Stabilized dispersion of insoluble sulfur and method of stabilizing

ABSTRACT

A method of achieving a novel superior dispersion of insoluble sulfur comprises mixing polysulfur while it is in the rubbery state with a diluent, such as rubber, to obtain a novel uniform dispersion of the sulfur in the diluent.

The present application is a division of application Ser. No. 08/995,770filed Dec. 22, 1997, now abandoned; which is a continuation-in-partapplication Ser. No. 08/187,659 filed Jan. 25, 1994, now U.S. Pat.No.5,475,059; which is a continuation-in-part of patent application Ser.No. 07/993,205 filed Dec. 18, 1992, now abandoned.

FIELD OF THE INVENTION

The invention relates generally to sulfur. More particularly, it relatesto a novel method of achieving novel superior dispersions of insolublesulfur in unvulcanized rubber by adding polysulfur to the rubber whilethe polysulfur is in the rubbery or unhardened state and to theresulting product.

BACKGROUND OF THE INVENTION

Rubber and similar polymers must be mixed with a selection of otheringredients to develop the properties necessary for specificapplications. One of these ingredients is a vulcanizing agent. Elementalsulfur is by far the most widely used vulcanizing agent, especially intires and other dynamic applications. While certain chemical compoundsof sulfur can be used as sulfur donors to accomplish vulcanization, onlyelemental sulfur is believed to impart the optimum combination ofproperties for most of the tire. One of the most important propertiesrequired in tire rubber is fatigue resistance. The superior fatigueresistance achieved when using elemental sulfur instead of sulfur donorsis reported in the “Natural Rubber Formulary” pp128-129, and again onpp180-181. Elemental sulfur is used in the rubber industry in two basicforms:

1. Ortho-rhombic (commonly called rhombic) crystals, consisting ofmolecules containing eight sulfur atoms per molecule in a ring-likestructure. This form is referred to as normal, or soluble sulfur. Itsmain disadvantage is that it “blooms” in the unvulcanized rubbercompound.

2. Polymeric sulfur (or polysulfur, as it is sometimes called todistinguish it from organic polymers containing sulfur in their polymerchains) consists of molecules that contain long chains of sulfur atoms,usually thousands of sulfur atoms per molecule. At room and processingtemperatures, these chains tend to revert to normal sulfur. Thisreversion can be deterred by adding certain stabilizing agents in smallquantities. The stabilized forms have dominated the market. Polymerizedsulfur or polysulfur is referred to as insoluble sulfur. There are noknown solvents for insoluble sulfur; hence its name. Its maindisadvantage is that it is hard to disperse well in unvulcanized rubbercompound. It is also quite expensive compared to normal sulfur.

The degree of dispersion, in the unvulcanized rubber compound, of all ofthese compounding ingredients affects the properties of the vulcanizedproduct. This is especially true of the vulcanizing agent. For the vastmajority of products, such as tires, the best dispersion gives the-bestproduct because of the homogeneity achieved.

Sulfur exists at room temperatures primarily as rhombic crystals. Otherforms of sulfur, such as monoclinic crystalline sulfur, or polysulfur,are the normal primary forms which elemental sulfur assumes at certainhigher temperature ranges. At room temperatures, these forms convert, orrevert, to rhombic sulfur.

Polysulfur is called insoluble sulfur, especially in the rubberindustry, and normal, non-polymeric or rhombic sulfur is called solublesulfur, because it is soluble to a limited extent in most rubbers. Theterm “rubber” as used herein means any sulfur vulcanizable polymer.Sulfur vulcanizable polymers are primarily those polymers having carbonto carbon molecular chain structures, with some double bonds existing intheir structure. These polymers are called unsaturated. The double bondsare the sites for sulfur vulcanization. The term “rubbery” as usedherein means masses of matter that are not hard, brittle, or friable,but are plastic and/or elastic. The term “saturated rubbery polymers”means those rubbery polymers that do not contain sulfur vulcanizablebonds, such as ethylene-propylene rubber.

The amount of normal or rhombic sulfur that is soluble in rubberincreases as the temperature increases. Typical rubber compounds containfrom one to three parts of sulfur per one hundred parts of rubberhydrocarbon (rhc). The processing of unvulcanized rubber requiresmechanical working of the rubber, which generates heat. The temperaturesdeveloped as a result of this processing are usually sufficient todissolve the typical normal sulfur content. When the rubber cools toroom temperature the solubility of the sulfur in rubber is exceeded, anda supersaturated solution ensues. This supersaturated portion of thesulfur tends to migrate to the surface of the rubber and crystallize.This condition is called “bloom” and is highly undesirable.

At room temperatures, surface blooming occurs primarily when theconcentration of soluble sulfur in the rubber is between the limits ofabout 0.8 parts and 8.0 parts per 100 parts of rubber hydrocarbon. Theselimits vary among different compounds. Below the lower limit the sulfuris soluble. Above the upper limit the sulfur drops out of solution inthe interior of most rubber compounds, forming micro-crystals throughoutthe mix. In some rubber compounds these micro-crystals grow toobjectionable size, causing nonhomogeneity of properties throughout thevulcanized product.

Polysulfur or insoluble sulfur does not dissolve in rubber, andtherefore does not bloom. However, the insoluble sulfur can revert tonormal sulfur, and the rate of reversion is a time-temperaturephenomenon which increases with temperature. Elemental insoluble sulfurcan be stabilized by the addition of various substances, notably thehalogens. This stabilized insoluble sulfur tends to remain polymeric atroom and processing temperatures but it reverts to normal sulfur at thehigher vulcanizing temperatures, thus becoming available for thevulcanization reaction.

Insoluble sulfur is normally supplied by the sulfur manufacturers indiscrete particles, or powder. This powder is extremely fine,classically having a reported average particle size of 3 microns. Theseparticles are considerably smaller than the particles usually suppliedof normal sulfur. These smaller particles are desired because thedispersion of this form of insoluble sulfur is limited by the particlesize supplied, unlike the dispersion of soluble sulfur. This very finepowder presents various processing difficulties. It tends to form dustclouds in the mixing room, which are both a health hazard and a safetyhazard. Sulfur dust explosions are a known hazard in the rubberindustry. A number of ways to reduce this dusting are mentioned in theliterature. Also, the sulfur powder is difficult to disperse in rubber.The individual particles tend to agglomerate. Because of this, thepowders are frequently mixed with a portion of a polymer or other matrixmaterials to form a masterbatch before being added to the finalcompound. These masterbatches usually contain fifty percent or moresulfur. This processing step adds to the cost. Since these discreteparticles retain their identity during mixing, the best possibledispersion is limited by the size of the particles, unless their meltingpoint is exceeded. However, when melted, the rate of reversion is veryrapid and the reverted sulfur, of course, blooms, and the advantages ofusing insoluble sulfur are negated.

The prior art falls in three categories:

1. Insoluble Sulfur Powders

U.S. Pat. No. 2,419,310 to Belchetz

U.S. Pat. No. 2,419,309 to Belchetz

U.S. Pat. No. 2,579,375 to Grove

These patents deal with insoluble sulfur in a form that has distinctdisadvantages. The present invention overcomes these disadvantages.

2. Sulfur Donors

U.S. Pat. No. 4,621,118 to Schloman

U.S. Pat. No. 2,989,513 to Hendry

U.S. Pat. No. 2,481,140 to Morris

All of these patents teach a chemical reaction of sulfur with an organiccompound to form sulfur donors. The crosslinking achieved using sulfurdonors is distinctly different from that achieved using elementalsulfur. No long chain polymers of sulfur are contemplated or achieved.Therefore they are not pertinent.

3. Solutions of Normal Sulfur

U.S. Pat. No. 1,782,693 to Miller

This patent teaches solutions of normal sulfur in an organic resin. Longchain polymers of sulfur do not go into solution in any known substance.Hence it fails to teach or suggest anything concerning polymeric sulfur.

Recently an “improved product” has been introduced, that has a reportedaverage particle size of 2 microns. The improvement is in the degree ofdispersion afforded by the smaller particle size. However, as could beexpected, the dusting problem has gotten worse with the “improvedproduct”, and the dispersion is, of course, still limited by theparticle size. There is still a need for much better dispersion than isachieved by the “improved product”, or that can be achieved by anyproduct that must rely on the size of a particulate to maximize itsdispersion.

It is generally known that rapidly cooling molten sulfur from asufficiently high temperature produces a mass of polymeric sulfur thatis in a rubbery or unhardened state. Sulfur in this rubbery state is ina metastable condition, and upon standing, it becomes hard and brittle.It can then be ground to form a powder. In some processes the moltensulfur is sprayed and thereby cooled, the individual droplets formed inthe spraying are also initially rubbery, but are then permitted tobecome hard and brittle. Special handling is necessary to keep them asindividual particles before hardening since the rubbery state tends tomake them agglomerate. In other processes, sulfur vapor is forced underpressure into a liquid cooling medium. Again rubbery particles are firstformed, and special handling is required until the particles harden. Therubbery unhardened state of insoluble sulfur has been consideredundesirable because it has been a free flowing powder which has beendesired.

I have discovered that the metastable, unhardened, rubbery or plasticstate of insoluble sulfur can be preserved by lowering the temperatureof the sulfur below the glass transition temperature of the rubberymass. Upon returning to room temperature, the mass again becomesrubbery, but upon further standing it becomes hard and brittle. When inthe rubbery state this sulfur can be dispersed in rubber to obtain thesuperior blends or dispersions of the present invention. If desired, thesulfur can be added to the rubber mix while it is still below its glasstransition temperature. When exposed to the milling temperature itquickly warms up to the rubbery state.

Other elements in Group VI_(B) of the Periodic Table of the Elements,notably Selenium and Tellurium, have many properties that are similar tosulfur. They have found limited use as vulcanizing agents for rubber,and they polymerize. It is well known that they polymerize themselvesand that they form copolymers and terpolymers with sulfur. Thesepolymers exhibit rubbery properties similar to those of the monopolymerof sulfur, and hence can be dispersed in similar fashions as the rubberypolymer of sulfur. All of these polymers are included in the term“polysulfur.”

Other elements, such as Arsenic, can be included in the sulfur polymerchain, and a rubbery state can still be obtained. One skilled in the artcould develop many such modifications of the sulfur polymer, and stillobtain a rubbery polymer. These modifications are included in thesubstance of this invention. The terms “polymeric sulfur” and“polysulfur,” as used herein, includes all of the modifications of thepolymer that can be obtained in the rubbery state.

There is a need for ways to achieve superior dispersion of polymericsulfur in rubber in a dust free manner. This need exists whether thesulfur is added to a rubber batch in the lower amounts needed for propervulcanization, or in the higher amounts used in preparing masterbatchesor intermediate batches, which are later added in the proportion neededto achieve the proper quantity of sulfur needed for vulcanization.

SUMMARY OF THE INVENTION

It is an object of the present invention to disclose how to achievesuperior dispersions of insoluble sulfur in unvulcanized rubber, in acompletely dust free manner.

It is a further object of the present invention to disclose novelsuperior blends made of insoluble sulfur with a diluent which iscompatible with sulfur vulcanizable polymers.

It is a further object to disclose a novel dust-free method of preparingsuch blends.

I have discovered that a superior dispersion of insoluble sulfur isattained if polysulfur is added to the rubber while the polysulfur is inthe metastable, unhardened or rubbery state. Previous to the presentinvention, this state has always been regarded as undesirable. Theliterature contains many descriptions of ways to harden this rubberymass, so as to make it usable by prior known art.

I have further discovered that superior preparations of polymericinsoluble sulfur can be prepared by a method which comprises mixingpolymeric sulfur while in the rubbery unhardened state with a compatiblediluent, which is also compatible with the polymer to be vulcanized. Theamount of polysulfur in the blend can vary from about 1% to about 95% byweight of the total blend. However, the preferred blends will containabout 20% to about 95% of the rubbery polysulfur by weight.

Electron Micrographs of a cross section of this superior dispersion,taken by a Leica Cambridge Ltd. S360 Scanning Electron Microscope,Version V0302A, equipped with Dynamic Stereo and Remote Control options,failed to discern any areas of unblended polysulfur. The same deviceclearly discerned the <1 micron to 3 micron particulates of CrystexSulfur, the brand of insoluble sulfur that presently dominates themarket because it has been the best available, in a 50/50 masterbatchdispersion of Crystex/EPR 707 (a high Mooney Viscosity EthylenePropylene Rubber). These micrographs also showed the non-uniformdispersion of the sulfur particulates, and showed three-dimensionalagglomerates of these particulates up to 29 microns across.

The blends of the present invention, since they are formed with thepolysulfur in the rubbery state, eliminate the need for comminution intofine powders, thereby effecting significant economies and eliminatingthe hazardous formation of dust. It also eliminates the need for therubbery sulfur polymer to harden, so that it can be processed into apowder, thereby effecting more economies. The blends of the presentinvention when properly prepared do not harden. They remain in theviscous or rubbery state so as to be readily dispersable by thecommercial methods used to mix rubber.

In the preferred practice of the method of the present invention, theconversion process by which soluble sulfur is converted to polysulfur isinterrupted when the polysulfur is in the rubbery state, and the rubberypolysulfur is mixed with the unvulcanized rubber, in the proportionneeded for proper vulcanization, or with the compatible diluent, at thattime.

In another embodiment, a portion of the polymer that is to be vulcanizedis mixed or masticated and used as the compatible diluent for therubbery polysulfur which is blended into it, thereby reducing the numberof processing steps even further. A two roll rubber mill or othermasticating equipment can be used.

In still another embodiment, normal or rhombic sulfur can be mixed in arelatively high percentage (e.g. 80%) with a compatible diluent, such asEPR (ethylene-propylene rubber, a saturated rubbery polymer) or anunsaturated rubbery polymer, and the sulfur polymerized in situ, byraising the temperature of the mixture to melt the sulfur and thenrapidly cooling the mixture below about 113° C. the melting point ofsulfur. This diluent is of such nature that it will not significantlyreact with the sulfur under the conditions of the process. This processis best done in a chambered mixer, such as an extended barrel extruderrather than on an open mill. When using the preferred high percentage ofsulfur, the molten sulfur tends to separate out of the mixture and runsoff the mill. In the preferred extruder, the first zone is heated andsubsequent zones cooled.

In another preferred embodiment, the sulfur conversion unit is set upconvenient to the intensive internal mixer of the Banbury type or arubber mill. The rubbery sulfur is added to the final or intermediatebatch in the quantities needed for proper vulcanization of the intendedfinal batch. A 1.6 kilogram (3 lb.) capacity laboratory mixer was used.

If desired, the polymerized molten sulfur can be added directly to therubber batch being mixed in the intensive mixer, using the rubber matrixas the rapid cooling medium to cool the mass and maintain it below themelting point of sulfur or to about 113° C. or less. The lower thetemperature the more stable and hence better the product will be.

It will be apparent to those skilled in the art that the above objectsand other advantages will accompany the practice of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the preferred practice of the method of the present invention,polysulfur in the rubbery state is blended with the compatible diluent,which is rubber, by mixing the rubbery polysulfur and the diluent on atwo roll rubber mill and continuing the blending or mastication until auniform blend is obtained. The amount of the rubbery polysulfur which isused is preferably enough to produce a uniform blend which containsabout 40% to about 80% of the polysulfur as based on the total weight ofthe blend.

The rubbery polysulfur can be prepared by any known manner. It is knownthat at about 113 degrees C. and above, sulfur melts. The molten sulfurbegins to polymerize as the temperature is raised to about 159 degreesC. This is evidenced by a pronounced increase in viscosity. As thetemperature is further increased, the viscosity increases sharply to amaximum in a rise of only a few degrees. It then drops off at a fairlysteady rate as the temperature is still further increased. It isbelieved that the mean chain length is at its maximum at the point ofmaximum viscosity, and that the mean chain length decreases as thetemperature rises further. The higher viscosities apparently have notbeen shown to inversely correlate with the percent of unpolymerizedsulfur present. The purer the sulfur, the higher is the maximumviscosity. The curve of the viscosity versus temperature of pure sulfuris reversible if the temperature changes are gradual. This reversibilitydoes not necessarily hold true when other substances, such as certainorganic compounds or halogens, are present in small amounts. In puresulfur, the end of the polymer chain is a free radical. When these othersubstances are present, either purposefully or accidentally, the freeradical is probably “capped” by the other substances. However, forpurposes of this application the term “polysulfur” is intended to coverthe just-described capped polymers.

If the molten sulfur is cooled gradually from the temperature of maximumviscosity, the drop in viscosity is attributed to the reversion of thepolysulfur to predominantly S₈. Other sulfur molecules, notably S₆, butothers from S₂ to S₂₃ have been identified.

When the molten sulfur is cooled rapidly from temperatures whereat asignificant portion of the sulfur is polymeric, the polymeric formsurvives to a metastable form, which is rubbery, at room temperature. Inpure sulfur, the polymer reverts to predominantly S₈ in a relativelyshort time. This reversion is hindered by the presence of halogens orcertain other substances, and seems to be accelerated by the presence ofmoisture, or alkalinity.

Upon rapid cooling, the molten polysulfur forms a rubbery mass ofrubbery polysulfur, which hardens by itself upon standing. Thishardening does not necessarily indicate de-polymerization, or reversion.Certain treatments can accelerate this hardening.

The compatible diluent which is blended with the rubbery polysulfur toform the blends of the present invention can be any substance which iscompatible with both the rubbery polysulfur and the polymer which is tobe vulcanized with the blend. Representative diluents include naturaland synthetic rubbers, soaps, petroleum fractions, waxes, wood tarproducts and plasticizers, such as ethers and esters and their polymers.

The method of preparing the blends can be any of those described herein,as well as other methods which can be used to form sufficiently uniformblends.

In the following examples 1 to 6, several techniques are described whichdemonstrate the wide variety of sulfur conversion processes that can beused in the practice of this invention.

EXAMPLE 1

One part of bromine is added to one hundred parts of sulfur in a glassretort. Heat is applied to melt and is then vaporize the sulfur. Whenthe distillate runs in a steady stream, which must be a very thinstream, it is run into a pot of ice water which is rotating atforty-five rpm. It is collected until the stream becomes unsteady, whichindicates that it will soon break. The sulfur forms a rubbery mass ofstrings in the ice water. It is removed from the ice water, and shook toremove most of the water. The remaining water is blown off withcompressed air. The rubbery polymeric sulfur is then blended with anequal amount of natural rubber, the compatible diluent, on a two rollmill. An excellent uniform blend of the polysulfur and the rubber isobtained.

EXAMPLE 2

The procedure of Example 1 is repeated except in place of 1 partbromine, 0.8 parts of bromine and 0.2 parts of iodine are used. Anexcellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 3

The procedure of Example 2 is repeated, but the distillate of polysulfuris run directly into the nip of a cooled two roll rubber mill upon whichthe rubber, the compatible polymer, has already been banded and hasformed a rolling nip. An excellent uniform blend of the polysulfur andthe rubber is obtained.

EXAMPLE 4

One hundred parts of sulfur and 0.25 parts of bromine are melted in abeaker and heated to a point (e.g. 200 degrees C.) above the maximumviscosity of the sulfur to form a liquid. The liquid is slowly pouredinto a rotating ice water bath to form rubbery polysulfur which afterisolation is blended with rubber, the compatible diluent, as inExample 1. An excellent uniform blend of the polysulfur and the rubberis obtained.

EXAMPLE 5

The procedure of Example 4 is repeated except that the molten liquidsulfur is fed into the compatible diluent, rubber, in the nip of therollers, as in Example 3. An excellent uniform blend of the polysulfurand the rubber is obtained.

EXAMPLE 6

One hundred parts of sulfur and one part of iodine are melted in analuminum dish on a hotplate and heated to spontaneous combustion. Themolten sulfur is sprayed with a hot spray gun onto a cool aluminum sheetto form a thin coating of rubbery polysulfur. The coating is peeled fromthe aluminum sheet, and laminated with a thin sheet of broken downrubber, the compatible diluent. The laminate is rolled up and passedthrough the nip of a two roll rubber mill. The mixture is cross rolledseveral times and banded on the mill. An excellent uniform blend of thepolysulfur and the rubber is obtained.

EXAMPLE 7

The uniform blends prepared as described in Examples 1-6 can be used asvulcanizing agents for sulfur vulcanizable polymers. The vulcanizingagents can be readily and uniformly dispersed in the polymer to bevulcanized. The resulting products are free of defects caused whensulfur is not uniformly dispersed in the compound.

EXAMPLE 8

Rubber Test Recipe parts per batch 100 RHC gms. per 3 lb. 1. NR 50.0420.0 2. SBR 50.0 420.0 3. HAF Carbon Black 50.0 420.0 4. Process Oil3.0 25.2 5. Zinc Oxide 3.0 25.2 6. Stearic Acid 2.0 16.8 7. Antioxidant1.0 8.4 8. Accelerator 1.0 8.4 9. Rubbery Sulfur 2.5 21.0 TOTAL 162.5pts. 1365.0 gms.

If it is desired to initially keep selected ingredients apart, such asthe sulfur and accelerator, two or more intermediate batches could bemixed separately, and later blended together, to give the desiredfinished batch formulation. Typical recipes for accomplishing this wouldbe:

Batch A Batch B 1. NR 50.0 50.0 2. SBR 50.0 50.0 3. HAF 50.0 50.0 4.Process Oil 3.0 3.0 5. Zinc Oxide 3.0 3.0 6. Stearic Acid 2.0 2.0 7.Antioxidant 1.0 1.0 8. Accelerator 2.0 0.0 9. Rubbery Sulfur 0.0 5.0TOTAL 161.0 164.0

Batch A and Batch B are each mixed. After cooling, they are blended inproportion to their batch weight to give the same finished batch as inthe formula immediately above.

EXAMPLE 9

95 parts of sulfur and 5 parts of Selenium are melted together andheated to 260° C. Upon rapid cooling, a rubbery mass is formed. This isthen blended with rubber. An excellent dispersion of this polysulfur,which contains Selenium, is obtained.

As previously described, the rubbery polysulfur used to make the blendsmay be prepared by any known method and it may or may not contain theatoms of the sulfur homologs of Group VI_(b)of the Periodic Table of theElements, notably Selenium and Tellurium. However, it also has beenfound that if rubbery polysulfur is treated, either after it is formed,or as it is formed, with a solvent for soluble sulfur, such as carbondisulfide, a chlorinated hydrocarbon, an aromatic hydrocarbon, or othersuitable solvent, it is possible to dissolve out some or substantiallyall of the soluble sulfur present. As a result, the polysulfur contentof the rubbery mass can thereby be increased on a percentage basis. As aresult, blends prepared from polysulfur thus treated contain lesssoluble or normal sulfur.

It will be apparent to those skilled in the art that a number ofmodifications and changes can be made without departing from the spiritand scope of the invention. Therefore, the invention is not to belimited except by the claims.

What is claimed is:
 1. A method of preparing a superior blend of polysulfur and rubber comprising adding the molten polysulfur at a temperature above about 159° C. to rubber as the rubber is being masticated, said rubber serving as a rapidly cooling medium, whereby the molten polysulfur is converted into the insoluble, rubbery form of polysulfur. 